The present invention relates to a novel method for detecting and counting foreign matter particles contained in a powder of a vinyl chloride-based polymeric resin as well as to a simple and efficient automatic apparatus system used therefor. More particularly, the invention relates to a method and an apparatus system for detecting and counting foreign matter particles contained in a powder of a vinyl chloride-based resin for the purpose of controlling the process conditions and product quality in the production of the resin powder products as well as for inspection of the base resin materials in the molding process of the resin powders into shaped articles.
As is known, inspection of a powder product of a vinyl chloride-based polymeric resin, referred to simply as a VC resin hereinafter, for detecting and counting foreign matter particles therein is conducted heretofore by either of the following two methods. Namely, the first of the methods is a simple visual inspection method of the resin powder per se in which a suitable amount of the resin powder taken by sampling is mounted as such on a suitable sheet such as a paper sheet or plastic sheet and the number of foreign matter particles therein is counted by naked eyes or under a magnifying glass. As a modification of this first method, it is also undertaken that the powder sample is spread over a sheet in a uniform thin layer by using a vibrator such as those sold under the tradename of Synton Vibrator before visual counting of the particles. The second of the conventional inspection methods is also a visual counting method but counting is conducted not for the resin powder as such but conducted for a light-transmitting shaped article prepared by molding the resin powder in advance in the form of a film, tape or plate.
The above mentioned first method totally relies on a man power labor so that reliability or reproducibility of the results cannot be ensured without extremely delicate and careful setting of the conditions of inspection procedure. Accordingly, the time required for inspection of one sample of the resin products cannot be short enough as desired in spite of the seeming simplicity of the method. The use of a vibrator is also not so effective to ensure high uniformity of the powder layer as spread thereby to be not recommendable when reproducibility of the results is essential. Alternatively, the sample resin powder is dropped in small portions onto a vibrating trough of a vibrator to run down on the trough in a powder stream of which the number of foreign matter particles is visually counted. This method is also not very efficient due to requirement for skillfulness of the inspection workers.
In the second method, it is of course that inspection of the molded article must be preceded by a molding process of the resin powder under inspection taking a considerable length of time so that this method is not applicable to the in-process control of the VC resin production process even by setting aside the problem of costs required for resin molding. In addition, this method is unavoidably not free from the troubles due to eventual entering of extraneous foreign matter particles from ambience leading to an incorrectly biased result of the inspection so that this method is questionable as an inspection method of VC resin powder products per se. At any rate, these prior art methods each heavily rely on the ability of the inspection workers including their eyesight power so that the reliability or reproducibility of the results of inspection inherently cannot be very high.
With an object to overcome the defects in the above described man-power methods for counting the number of foreign matter particles in a VC resin powder, Japanese Patent Kokai 2-49147 proposes an automatic inspection machine for counting foreign matter particles in a VC resin powder comprising a means for removal of static electricity from the white sample powder, a hopper to receive the sample powder, a continuous vibration feeder means of the sample powder provided below the hopper, a transfer means for the sample powder provided below the discharging end of the feeder means, a gas laser scanning detector of the reflection type provided above the transfer means and a processing and indicating means of the data obtained in the detector, in which the sample powder after removal of static electricity is mounted continuously under vibration onto the transfer means to form a flat powder layer of a uniform thickness and then the foreign matter particles in the powder layer are detected and counted by means of the gas laser scanning detector.
In the inspection method by using this machine, it is necessary in order to form a flat powder layer of a uniform thickness that the VC resin powder after removal of static electricity is moistened with steam, water, surface active agent or alcoholic solvent so that the resin particles is kept in a freely dispersible condition without leaving any undispersed lumps. This requirement for dispersibility of the sample powder can of course be fulfilled only by taking a considerable length of time for the moistening works. In addition, it is a rather difficult matter to obtain a thin powder layer of a uniform thickness of 0.7 to 1.5 mm even after removal of static electricity from the VC resin powder. Accordingly, this method is also not quite efficient when a large number of VC resin powder samples are to be subjected to inspection of foreign matter particles as a daily routine work.
The importance of the above described inspection methods for counting foreign matter particles in a VC resin product is increasing in recent years to be in compliance with the progress of the VC resin producing process which is under a trend of switching from the traditional batch-wise process of suspension polymerization of vinyl chloride monomer to a continuous polymerization process through a closed polymerization reactor which is not opened over a quite long time for continuous running of polymerization. Needless to say, such a long-term continuous running of the polymerization process without intermission can be accomplished only by overcoming the most difficult problem due to deposition of resin scale on the inner surface of the reactor walls. While the traditional and most effective method for prevention of scale deposition is to coat the reactor walls with a scale deposition inhibiting agent and long-life scale deposition inhibiting agents have been developed for use in a continuous polymerization reactor, it is unavoidable, however, that the effectiveness of the scale deposition inhibiting coating layer is subject to a gradual decrease in the long run resulting in a gradually increasing amount of deposited resin scale on the reactor walls, which eventually falls off the reactor walls to enter the VC resin product as foreign matter particles in addition to the scale debris and other metallic and inorganic dust particles left on the reactor walls due to incomplete cleaning of the reactor periodically undertaken by opening the reactor.
It is eagerly desired therefore in the industry producing or using VC resin products to develop a convenient and efficient method and an apparatus system for detecting and counting foreign matter particles contained in a VC resin powder capable of giving reliable and reproducible results without taking a long time per one powder sample under inspection.